MATING TYPE SILENCING AND SWITCHING IN YEAST

酿酒酵母的交配型基因座提供了一个引人注目的 仓位效应调节的例子。交配型调控基因 位于酵母III染色体末端的基因座，命名为HML 和HMR，受到一种新的监管体系的抑制，该体系包括 “消音器”调节四个非连锁基因产物的作用， SIR1至SIR4，以在消音器附近呈现DNA 一般难转录的。我们已经证明了DNA跨越 无声的盒式磁带包装在去乙酰化的核小体中 相对于那些在基因组活跃部分的基因，这 低乙酰化是由SIR基因本身的产物催化的。 此外，我们还定义了消音器的显著特征 位于HML，并已表明他们协同行动可能是为了指导 无声的盒式磁带上的染色质凝结。我们建议(1) 使用低乙酰化作为标签来定义 沉默染色质及其与结构和取向的关系 (2)研究组蛋白H1和H5同系物的作用 在沉默和评估设施中，染色质从沉默 区域可以经历到凝聚态的转变；(3)为了识别， 克隆和鉴定与沉默有关的新基因。 我们还建议研究潜在的分子机制 酵母交配型相互转化的方向性。激活 两个静音配对盒式磁带中的信息通过定向发生 转位到一个表达位点垫上。供体盒的选择 在任何一次换位中都不是随机的，而是由交配决定的 等位基因出现在垫子上。这就产生了精确的图案顺序 交配型相互转换。我们计划研究其背后的机制 通过识别顺式和顺式的交配型相互转换的方向性 选择供体基因座所需的反式组分，至(1) 我们先前分离的反式作用突变体的特征 在选择配对类型的盒式磁带时失去了正常的偏向；和(2) 互换HML和HMR盒式磁带以及其不同程度的 侧翼序列，以识别细胞用来 区分这两个基因座。

The mating type loci in Saccharomyces cerevisiae provide a striking example of position effect regulation. Mating type regulatory genes located at the ends of chromosome III of yeast in loci, designated HML and HMR, are repressed by a novel regulatory system comprised of "silencers" that mediate the action of products of four unlinked genes, SIR1 through SIR4, to render DNA in the vicinity of the silencers generally refractory to transcription. We have shown that DNA spanning the silent cassettes is packaged in nucleosomes that are hypoacetylated relative to those in active portions of the genome and that this hypoacetylation is catalyzed by the products of the SIR genes themselves. In addition, we have defined the salient features of the silencers located at HML and have shown that they act in tandem probably to direct chromatin condensation across the silent cassettes. We propose (1) to use hypoacetylation as a tag to define the extent and organization of silenced chromatin and its relationship to the structure and orientation of the silencers; (2) to examine the role of histones H1 and H5 homologs in silencing and assess the facility with which chromatin from silenced regions can undergo transition into a condensed state; (3) to identify, clone and characterize novel genes involved in silencing. We also propose to examine the molecular mechanism underlying directionality of mating type interconversion in yeast. Activation of information within the two silent mating cassettes occurs by directed transposition to an expressor locus MAT. Selection of the donor cassette during any one transposition is not random but is dictated by the mating allele present at MAT. This yields a precisely patterned order for mating type interconversion. We plan to examine the mechanism underlying this directionality of mating type interconversion by identifying cis and trans components required for selection of the donor locus, through (1) characterization of trans-acting mutants we previously isolated that have lost the normal bias in selecting mating type cassettes; and (2) interchanging HML and HMR cassettes along with varying extents of their flanking sequences in order to identify sites used by the cell to distinguish the two loci.